Over the last two decades, scientific studies have revealed the negative impacts of certain types of artificial light on health. New lamps using light-emitting diodes (LEDs) address these findings, adjusting color temperature and flicker to produce a light that reduces negative health impacts.
Human circadian rhythms are known to be disturbed by stimuli from modern technologies. Electronic-device screens expose humans to relatively intense sources of blue and green light at all hours. Before electric light, humans were exposed to only natural sources of light, namely sunlight, moonlight and firelight, but artificial light has reduced the hours spent in sunlight, further disrupting circadian rhythms.
Current lighting products purport to reduce sleep-disturbing blue light but may still produce unhealthy levels of flicker.
Lumens (lm) measure the perceived power of a light source weighted to human vision. This metric is commonly used to define the intensity of a light bulb. A 60-watt incandescent source produces about 650-800 lumens (lm); a 40-watt source produces approximately 400-450 lm.
Luminous flux, measured in lumens (lm), defines the intensity of light produced by a source, while illuminance, measured in lux, is the intensity of light received at the eyes. Lux (lx) is the SI measurement unit of illuminance, measuring luminous flux per area (lumens per square meter) on a surface.
While light levels are commonly specified and measured on the horizontal plane, modern electronic displays are typically oriented as vertical surfaces.
When we stand or sit upright, our eyes perceive light on the vertical plane, and this is how circadian light levels are measured.
Blue-green light stimulates circadian rhythm. The protein melanopsin is activated in the presence of blue and green light.
Equivalent melanopic lux (EML) is a measure of illuminance weighted to the blue-green sensitivity of melanopsin. EML is used to quantify circadian light and is measured on a vertical plane. Illuminance is the total luminous flux per area incident on a surface.
Melanopic lumens are a measure of luminous flux weighted to melanopsin sensitivity. Melanopic lumens can be used to quantify the circadian output of a light source.
Correlated Color Temperature (CCT, or Color Temperature) of a light source is its temperature, expressed in degrees Kelvin (K). A low-CCT LED tends to have a relatively lower luminous efficacy (efficiency) than a high-CCT LED. An evening reading light source should have a nominal CCT between 1800 Kelvin and 2400 Kelvin, to approximate traditional light sources during and post sundown.
“Color rendering” describes a light source's accuracy in rendering reference color samples. The Color-Rendering Index (CRI Ra) is a quantitative measure of a light source's capacity to reveal colors faithfully in comparison with an ideal light source. Sunlight, incandescent, halogen, and some specialized LED sources have CRIs in the high 90s. Modern fluorescent lighting has values in the 80s or 90s. LED lighting typically has a CRI around 80.
TM-30 is a lighting-fidelity metric and and more-descriptive alternative to CRI Ra. Rf is the color rendering fidelity index value for TM-30, which ranges from 0-100. By comparing the 99 TM-30 reference color samples to the rendering of the light source, Rf is computed. A score of 100 is considered to have perfect rendering for all samples.
Rg is another TM-30 metric. An Rg score of 100 means that the light source is, on average, neutrally saturated. Rg from 90-110 is considered very good for general lighting.
Levels of red are expressed in R9, a metric that is not included in calculating CRI Ra. Studies have shown that humans prefer light with high amounts of red. A desirable R9 is 50 or above. Incandescent, halogen, and daylight have R9s of 70 or above. A typical LED source has an R9 close to zero, or even negative.
“Flicker” is rapid, repeated changes in light intensity over time. It is usually invisible and is part of the normal operation of a light source. Magnetic fluorescent ballasts were subject to flicker criticism, but electronic ballasts introduced in the 90s, as well as compact fluorescent lamps, pulse at high speeds (10s of kHz) and have largely solved the flicker problem.
Cheaply-produced LED lights can flicker.
Flicker may be detectable in the presence of moving objects or in the peripheral vision and is known to contribute to headaches, eyestrain and reduced concentration. It is demonstrated in numerous scientific experiments that under flickering light, saccades, or eye movements between two points, do not travel the same distance that they would under steady light. The eyes will overshoot or undershoot the target.
IEEE 1789-2015 is a standard that defines two important flicker metrics: the modulation percentage, which is the interval between the minimum and maximum height of an oscillation; and flicker frequency, which is the frequency of the oscillation in Hz. Because of potential negative consequences of flicker, good lighting schemes are designed to reduce or eliminate it.
Bedtime lighting is designed to minimize circadian input while providing sufficient light for reading. A commonly-recommended indoor task-light level is 500 lx on the horizontal plane. Most home task lighting is between 300 and 700 lx, which has been determined to be uncomfortably bright for the evening. An evening-reading lighting level of 100-200 lx in the horizontal plane is considered ideal for most people.
An evening-reading light source should produce no more than 200 melanopic lumens. This is much lower than a typical 650-800 lumen “soft white” incandescent, halogen, or LED light source, which produces between 330 and 450 melanopic lumens.
A light source ranging from 300 to 450 lumens will produce 100-200 lx in the horizontal plane at 2-4 feet. This is the case for unshaded lamps, in lamps with white and beige shades, and in lamps with white frosted and gray translucent glass globes.
Intrinsically Photosensitive Retinal Ganglion Cells (ipRGCs) are photoreceptors, distinct from rods and cones, which are sensitive to blue and green light. ipRGCs are not used for vision, but they provide an input the circadian rhythm via the retinohypothalamic tract.
LEDs, or light-emitting diodes, are semiconductor devices that emit light when powered.
“A 19” represents a standard light bulb shape, with “19” representing bulb's widest diameter (19/8 inches). A common base for an A19 lamp is the E26 screw (E for Edison screw, 26 mm) in North America. It may be an E27 screw (27 mm), B22 bayonet (Bayonet, 22 mm), or another shape.
“B10” refers to a pointed bulb shape, often referred to as “candelabra,” with a diameter of 10/8 inches. B10 lamps typically have an E12 or E26 base.
The directional emission characteristics of LED lamps can cause emitted shadows. Light from an LED lamp directed at a surface may have shadows cast within the diffused light depending on the arrangement of the LEDs in the lamp.